Amphiphilic Hyperbranched Polyethoxysiloxane: A Self-Templating Assembled Platform to Fabricate Functionalized Mesostructured Silicas for Aqueous Enantioselective Reactions

Article abstract of DOI:10.1021/acscatal.6b01315

Fabrication of amphiphilic mesostructured silica as a heterogeneous catalyst is beneficial to facilitate an aqueous reaction due to its highly dispersed nature in water. In this work, by taking advantage of a self-templating assembled strategy, we constru

Full text of DOI:10.1021/acscatal.6b01315

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Article
Amphiphilic Hyperbranched Polyethoxysiloxane: A Self-
Templating Assembled Platform to Fabricate Functionalized
Mesostructured Silicas for Aqueous Enantioselective Reactions
Kun Zhang, Juzeng An, Yanchao Su, Jueyu Zhang, Ziyun Wang, Tanyu Cheng, and Guohua Liu
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01315 • Publication Date (Web): 11 Aug 2016
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Amphiphilic Hyperbranched Polyethoxysiloxane: A Self-
Templating Assembled Platform to Fabricate Functionalized
Mesostructured Silicas for Aqueous Enantioselective Reac-
tions
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Kun Zhang, Juzeng An, Yanchao Su, Jueyu Zhang, Ziyun Wang, Tanyu Cheng, Guohua Liu*
Key Laboratory of Resource Chemistry of Ministry of Education, Shanghai Key Laboratory of Rare Earth Functional Materiꢀ
als, Shanghai Normal University, Shanghai, China. Tel: + 86 21 64322280; Eꢀmail: ghliu@shnu.edu.cn.
ABSTRACT: Fabrication of amphiphilic mesostructured silica as a heterogeneous catalyst is beneficial to facilitate an aqueous
reaction due to its highly dispersed nature in water. In this work, by taking advantage of a selfꢀtemplating assembled strategy, we
construct two types of mesostructured silicas as
S
Cat*
heterogeneous catalysts, chiral ruthenium/diamineꢀ
functionalized and chiral cinchonineꢀbased
squaramideꢀfunctionalized heterogeneous catalysts,
through the use of amphiphilic poly(ethyleneglycol)
O
R
O
Si
Cat*
Cat*
R
Ru
N
CF3
Cl
N
R
NH
H
NSO2
Ph
H2N
H
Cat*
OMe
CF3
monomethyletherꢀmodified
hyperbranched
O
5
Catalyst
Cat*
Ph
N
O
R =
polyethoxysiloxane as a silica precursor. As presented
in the study, chiral ruthenium/diamineꢀfunctionalized
7
Catalyst 3
O
O
R1
R1
O
O
N
HN
3
Catalyst
NO2
catalyst
performs
an
asymmetric
transfer
Ar
NO2
Catalyst 5
hydrogenation of acyclic αꢀtrifluoromethylimines to
chiral αꢀtrifluoromethylamines in water while chiral
squaramideꢀfunctionalized one enables efficiently
asymmetric Michael addition of acetylacetone to
HCOONa
Ar
CF3
Ar
CF3
Ar
15 samples
up to 98% ee
11 samples
up to 99% ee
nitroalkenes in brine. Both highly catalytic performances are attributed to the combined multifunctionalities of wellꢀdefined singleꢀ
site chiral active species, highly dispersed catalytic centers and practical phaseꢀtransferred function. Furthermore, both catalysts can
also be recovered easily and reused repeatedly for at least seven times without loss of catalytic activity. Such a feature makes this
selfꢀtemplating assembly attracting in construction of various heterogeneous catalysts.
KEYWORDS. Asymmetric catalysis, heterogeneous catalyst, immobilization, mesoporous material, silica.
templating assembled process, which simplifies greatly the
synthetic procedure of heterogeneous catalysts. In particular,
INTRODUCTION
Development of silicaꢀbased immobilizated strategy to
fabricate heterogeneous catalysts for enantioselective reactions
has obtained great achievements in heterogeneous asymmetric
catalysis, where numerous reviews have been wellꢀ
documented and some have exhibited highly catalytic efficienꢀ
cy.1 Although many silicaꢀbased assembled methods have
been explored in construction of various heterogeneous
catalysts,2 direct utilization of a selfꢀtemplating assembled
approach3 to fabricate a functionalized mesostructured silica
as a heterogeneous catalyst for an enantioselective reaction is
still rare thus highly desirable. A significant benefit of such a
selfꢀtemplating assembled approach is that various chiral funcꢀ
tionalities could be incorporated easily into silicate networks,
which is not only simple but also is effective in construction
of heterogeneous catalysts. Furthermore, no templateꢀdirecting
assembled reagent means no extractionꢀstep during a selfꢀ
an obtained heterogeneous catalyst through a selfꢀtemplating
assembly without any templateꢀdirecting reagents can reflect
truly catalytic nature of heterogeneous catalyst itself, as this
selfꢀtemplating assembly eliminates completely the disturbꢀ
ance coming from those residual structureꢀdirecting reagents
that are employed generally and are difficult to be removed
during an assembled method.4
As an
poly(ethyleneglycol)
important amphiphilic silica precursor,
monomethyletherꢀmodified hyperꢀ
branched polyethoxysiloxane (PEGꢀPEOS), possesses a selfꢀ
templating ability to construct various mesoporous materials
without any templateꢀdirecting reagents.5 One pioneering work
reported by Zhu and Möller group utilizes this amphiphilic
PEGꢀPEOS molecule under basic conditions to assemble
various silica materials with controlled morphology, where
mesoporous particles, hollow nanospheres and ultrasmall solid
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particles could be formed through the adjustment of amounts
3432.6 (s), 3057.8 (w), 2858.2 (w), 1633.7 (m), 1499.8 (w),
1458.6 (w), 1392.1 (w), 1325.5 (w), 1167.2 (s), 1092.2 (s),
950.7 (m), 800.8 (m), 700.6 (m), 567.5 (m), 467.4 (m). 13
of hydrophilic PEG fragments in PEGꢀPEOS molecule.5d More
importantly, the amounts of PEGꢀfunctionality in a selfꢀ
templating assembled process can be fineꢀtuned. This feature
is beneficial to form an adjustable PEGꢀfunctionalized mesoꢀ
porous silica. Meanwhile, these PEGꢀfunctionalities possess a
significant benefit in catalysis because they can boost an
aqueous catalytic performance due to their additional phaseꢀ
transfer function in an aqueous reaction condition. In addition,
scalable preparation and facile functionality of PEGꢀPEOS
have opened up opportunities in fabrication of various funcꢀ
tionalized materials, where selfꢀtemplating assembly of PEGꢀ
PEOS has produced successfully various functionalized
nanocapsules for medicine delivery and encapsulation of enꢀ
zymes.6 However, to the best of our knowledge, these unique
advantages applied in catalysis are not still explored yet. Thus,
by utilizing the advantage of amphiphilic mesoporous silica
developed by Zhu and Möller group, incorporation of chiral
siloxane into amphiphilic PGEꢀPEOSꢀbased silicate network
not only constructs an amphiphilic chiral moleculeꢀ
functionalized mesostructured silica as a heterogeneous cataꢀ
lyst through a selfꢀtemplating assembled strategy but also can
realize a highly efficient asymmetric catalysis in water, which
is of great interest both fundamentally and practically.
C
CP/MAS NMR (161.9 MHz): 147.8, 138.2, 128.1 (C of Ph
and Ar groups), 105.0, 102.7 (C of mesitylene ring),
76.3−71.0 (CH of −NCHPh), 65.5−56.5 (CH2 of
−OCH2CH2O−), 29.9 (CH2 of −CH2Ar), 23.2−12.5 (CH3 of
mesitylene and CH3 of –CH2CH3), 9.1 (C of −CH2Si) ppm.
29Si MASNMR (79.4 MHz): T2 (δ = −59.8 ppm), T3 (δ =
−69.3 ppm), Q3 (δ = −102.5) ppm, Q4 (δ = −112.2 ppm).
General procedure for asymmetric reaction. A typical proꢀ
cedure was as follows. The catalyst 3 (19.56 mg, 4.0 ꢁmol of
Ru, based on ICP analysis, 2.0 mol%), (αꢀ
trifluoromethylimines (0.20 mmol), HCOONa (68.0 mg, 1.0
mmol, 5.0 equiv) and 2.0 mL of H2O were added sequentially
to a 5.0 mL round−bottom flask. The mixture was then stirred
at 40 °C for 12−24 h. During this period, the reaction was
monitored constantly by TLC. After completion of the reaction,
the catalyst was separated by centrifugation (10,000 rpm) for
the recycling experiment. The aqueous solution was extracted
with ethyl ether (3 × 3.0 mL). The combined ethyl ether exꢀ
tracts were washed with NaHCO3 and brine, and then dehyꢀ
drated with Na2SO4. After evaporation of ethyl ether, the resiꢀ
due was purified by silica gel flash column chromatography to
afford the desired product. The yields were determined by 1Hꢀ
NMR, and the ee values were determined by a HPLC analysis
using a UVꢀVis detector and a Daicel OD–H or OB–H or AD–
H chiralcel column (Φ 0.46 × 25 cm).
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As efforts to construct various silicaꢀbased heterogeneous
catalysts for enantioselective reactions,7 in this contribution,
we employ an amphiphilic PEGꢀPEOS as a silica precursor
and a selfꢀtemplating strategy as an assembled approach, conꢀ
structing two types of chiral ruthenium/diamineꢀfunctionalized
and chiral cinchonineꢀbased squaramideꢀfunctionalized
mesostructured silicas as heterogeneous catalysts. The adꢀ
vantage of this approach is a facile selfꢀtemplating sol–gel
process without any additives and the benefit of both heteroꢀ
geneous catalysts is efficient in enantioselective reactions in
an environmentally friendly medium.
RESULTS AND DISCUSSION
Synthesis and structural characterization of the
heterogeneous catalyst 3
EXERIMENTAL SECTION
Preparation of of the heterogeneous Catalyst 3. In a typical
procedure, 1.0 g (0.36 mmol) of PEGꢀPEOS was dispersed in
20.0 mL of deionized water and was treated by ultrasonic irraꢀ
diation for 10 min. Then an ammonia aqueous solution (25%)
(1.20 mL) was added to adjust the ammonia concentration to
0.7 M. The mixture was kept under gentle stirring (500 rpm) at
25 °C for 3 h. After that, 0.10 g (0.20 mmol) of (S,S)ꢀ
ArDPENꢀderived silica (2) was added to the solution. The
reaction mixture was stirred at 25 °C with a stirring speed of
500 rpm for another 24 h. The solids were collected by cenꢀ
trifugation and washed repeatedly with excess distilled water.
The solid was filtered, rinsed with ethanol again, and then
dried at 60 °C under reduced pressure overnight to afford
SCHEME 1. Preparation of the heterogeneous catalyst 3.
ArDPENꢀfunctionalized
mesostructured
nanoparticles
Selfꢀtemplating assembly of chiral MesityleneRuArDPENꢀ
(ArDPENꢀMSNs) (0.65 g) in the form of a white powder. The
collected solids (0.50 g) were suspended in 20.0 mL of dry
CH2Cl2 again and 0.10 g of (0.17 mmol) [RuCl2(mesitylene)]2
were added. The resulting mixture was stirred at 25 °C for 24
h. The mixture was filtered through filter paper and then
rinsed with excess CH2Cl2. After Soxhlet extraction for 10 h in
CH2Cl2 to remove homogeneous and unreacted starting mateꢀ
rials, the solid was dried at ambient temperature under vacuum
overnight to afford catalyst 3 (0.45 g) as a light−yellow powꢀ
der. ICP analysis showed that the Ru loadingꢀamount was
20.65 mg (0.204 mmol) per gram catalyst. IR (KBr) cm−1:
functionality
mesitylene)RuC1[Nꢀ((S,S)ꢀ2ꢀaminoꢀ1,2ꢀdiphenylethyl)ꢀ4ꢀ
(MesityleneRuArDPEN:8
((η6ꢀ
ethylbenzenesulfonamide], where ArDPEN = Nꢀ((1S,2S)ꢀ2ꢀ
aminoꢀ1,2ꢀdiphenylethyl)ꢀ4ꢀethylbenzenesulfonamide) within
the PEGꢀPEOSꢀbased silicate network produced the
ruthenium/diamineꢀfunctionalized mesostructured nanopartiꢀ
cles, abbreviated as MesityleneRuArDPENꢀMSNs (3), as outꢀ
lined in Scheme 1. Firstly, the amphiphilic silica precursor
PEGꢀPEOS (1) was synthesized according to the reported
method.5 The selfꢀtemplating assembly of 1 and 2 then gave
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the chiral ArDPENꢀfunctionalized mesostructured nanopartiꢀ
species, respectively. These observations demonstrated that
cles (ArDPENꢀMSNs), which was coordinated with
[RuCl2(mesitylene)]2 to form crude heterogeneous catalyst.
Finally, this crude heterogeneous was treated with a Soxhlet
extraction, affording the pure catalyst 3 as a lightꢀyellow powꢀ
der (see SI in Figures S1ꢀS5).
catalyst 3 possessed mainly the inorganosilicate networks of
(OH)Si(OSi)3 and Si(OSi)4 with the organosilicate R–Si(OSi)3
species as its main part of silica walls.9
3
4
5
6
7
Singleꢀsite chiral MesityleneRuArDPEN active center inꢀ
corporated in the silicate network of 3 could be confirmed by
solidꢀstate 13C CP/ MAS NMR spectroscopy. As shown in
Figure 1, catalyst 3 showed the carbon signals around ~74
ppm and ~128 ppm, which were corresponded to carbon atꢀ
oms of the –NCHC6H5 groups in ArDPEN moiety. Peaks at
105.0 and 102.7 ppm in the spectrum of 3 ascribed the characꢀ
teristic carbon atoms of the aromatic ring in mesitylene moiety,
whilst peak at 20.8 ppm was attributed to the characteristic
carbon atom of the CH3 groups attached to the aromatic ring in
mesitylene moiety. However, these observed characteristic
peaks in the spectrum of its parent ArDPENꢀMSNs were abꢀ
sent, indicating the formation of the chiral MesityleneRuꢀ
ArDPEN complex as a singleꢀsite active center. Furthermore,
the chemical shift values in catalyst 3 were similar to those of
its homogeneous MesityleneRuTsDPEN,8 elucidating both
wellꢀdefined singleꢀsite chiral active species. This judgment
could be validated by a XPS investigation (see SI in Figure
S2), where catalyst 3 had the same Ru 3d5/2 electron binding
energy as its homogeneous MesityleneRuTsDPEN (281.41 eV
versus 281.44 eV). These findings confirmed that immobilizaꢀ
tion of chiral MesityleneRuTsDPENꢀfunctionality within its
PEGꢀPEOSꢀbased silicate network through a selfꢀtemplating
sol–gel process followed by a complexation approach could
retain the original chemical environment of its homogeneous
MesityleneRuTsDPEN, which is crucial to control its enantiꢀ
oselective performance discussed below.
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g
ArDPENꢀMSNs
Catalyst 3
*
a
g
b
c
*
d
g
e
FIGURE 2. (a) SEM images of 3, (b) TEM images of 3, and (c)
SEM image with a chemical mapping of 3 showing the distribuꢀ
tion of Si (white) and Ru (red), (d and e) the dispersive situations
in water as indicated catalyst 3 (d) and its parallel analogue 3' (e).
f
250
200
150
100
50
0
ppm
The morphology, pore structure and ruthenium distribution
of catalyst 3 were further investigated by means of scanning
electron microscopy (SEM), transmission electron microscopy
(TEM), and nitrogen adsorptionꢀdesorption techniques. As
shown in Figure 2, the SEM image (Figure 2a) revealed that
catalyst 3 was composed of the monodisperse nanoparticles
while the TEM image (Figure 2b) demonstrated its mesostrucꢀ
ture confirmed by its nitrogen adsorptionꢀdesorption isotherm
of catalyst 3 due to its typical IV character with an H1 hystereꢀ
sis loop (see SI in Figure S4), where both observations were
similar to those of its corresponding pure material.5d In particꢀ
ular, the TEM image with a chemical mapping technique disꢀ
closed that the ruthenium centers were uniformly distributed
within its silicate network (Figure 2c). This finding, together
FIGURE 1. The solidꢀstate 13C MAS NMR spectra of ArDPENꢀ
MSNs and catalyst 3.
The 29Si MAS NMR spectrum further demonstrated the
main silicate compositions of 3 (see SI in Figure S3), where
catalyst 3 presented two groups of typical silica signals (Q
signals for inorganosilica and T signals for organosilica). As
compared these values with those typical ones in the literaꢀ
ture,9 the strong T3 signal at −68.9 ppm was attributed to R–
Si(OSi)3 species (R = alkylꢀlinked PEGꢀfunctionality and Meꢀ
sityleneRuArDPENꢀfunctionality), while the Q3−Q4 signals at
−102.3 and −110.2 ppm ascribed (HO)Si(OSi)3 and Si(OSi)4
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3
4
with the highly dispersive nature in water (catalyst 3 relative to
its parallel analogue 3' in Figures 2dꢀ2e), will govern its catalytꢀ
ic performance discussed below.
12
13
14
15
16
Ph
pꢀMePh 16
pꢀMePh 16
pꢀMePh 16
90
93
90
95
92
91
96
96
95
94
94
96
pꢀFPh
pꢀBrPh
Ph
These characterizations and analyses expatiate that amꢀ
phiphilic poly(ethyleneglycol) monomethyletherꢀmodified
hyperbranched polyethoxysiloxane as a selfꢀtemplating silica
precursor could assemble steadily a heterogeneous catalyst,
where the wellꢀdefined singleꢀsite chiral ruthenium/diamineꢀ
functionality and PEGꢀfunctionality were incorporated into the
silicate network of monodisperse nanoparticles.
5
6
7
8
Ph
18
12
9
Ph
pꢀClPh
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17
Ph
1ꢀnaphthyl 18
Catalytic property of the heterogeneous catalyst
a
Reaction conditions: catalyst 3 (19.56 mg, 4.0 ꢁmol of Ru based on
ICP analysis), αꢀtrifluoromethylimines (0.20 mmol), 1.0 mmol of
Chiral αꢀtrifluoromethylamines as an important motif can be
transferred into various biologically active molecules in mediꢀ
cal and fluorine chemistry,10 where direct construction of these
chiral αꢀtrifluoromethylamines through asymmetric transfer
hydrogentation (ATH) have also been involved.7a, 11, 12 With
the heterogeneous catalyst 3 on hand, we investigated its ATH
HCOONa in 2.0 mL of water, and reaction time (12ꢀ28 h). b The ee values
c
were determined chiral HPLC analysis (see SI in Figure S8). Data were
obtained using the homogeneous MesityleneRuTsDPEN as catalyst.
Data were obtained using the mixed PEG plus the homogeneous Mesityꢀ
leneRuTsDPEN as catalyst. Data were obtained using its parallel anaꢀ
d
e
logue 3' as a catalyst.
in
water
using
4ꢀmethoxyꢀNꢀ(2,2,2ꢀtrifluoroꢀ1ꢀ
Based on its highly catalytic performance in the ATH of 4ꢀ
methoxyꢀNꢀ(2,2,2ꢀtrifluoroꢀ1ꢀphenylethylidene)aniline,
phenylethylidene)aniline as a model reaction at first, where the
reaction was carried out with HCO2Na as a hydrogen source
and 2.0 mol% of 3 as a catalyst according to the reported
method.7a It was found that the ATH of 4ꢀmethoxyꢀNꢀ(2,2,2ꢀ
trifluoroꢀ1ꢀphenylethylidene)aniline catalyzed by 3 gave (S)ꢀ4ꢀ
methoxyꢀNꢀ(2,2,2ꢀtrifluoroꢀ1ꢀphenylethyl)aniline with 95%
yield and 96% ee. Such an ee value was markedly better than
that attained with its homogeneous counterpart (Mesityleneꢀ
RuTsDPEN) in water (Entry 1 vs Entry 1 in brackets),
suggesting the amphiphilic benefit of the designed catalyst 3.
a
series of aryl–substituted substrates were further investigated
under the same reaction conditions. As shown in Table 1, it
was found that all tested substrates could be transferred into
the responding chiral aryltrifluoromethylamines with high
yields and enantioselectivities. It was noteworthy that the
structural and electronic properties of substituents in the
aromatic ring at R2 group did not affect significantly their
enantioselectivities, where the reactions with various electron–
withdrawing and electron–donating substituents in the
aromatic ring at R2 group were equally efficient (Entries 4–10).
But the slightly effects on yields could be observed, which the
reactions with electron–withdrawing substituents in the Ar
moiety at R2 group had slightly higher yields than those of
electron–donating substituents (Entries 4–8 versus entries 9ꢀ
10). In addition, the thienylꢀsubstituted substrate could be also
converted in a highly enantioselective performance (Entry 11).
Besides the general pꢀmethoxyphenyl (PMP) protection group,
other protection groups, such as pꢀtolyl, Ph, and 1ꢀnaphthyl,
could also be used in this asymmetric reaction with high
enantioselectivities (Entries 12–17).
TABLE 1. Asymmetric transfer hydrogenation of αꢀ
trifluoromethylimines.a
Time
(h)
Yield
(%)b
Ee.
Entry R1
R2
(%)b
1
Ph
PMP
PMP
PMP
PMP
PMP
PMP
PMP
PMP
PMP
PMP
PMP
16(24)
16
96(89)c
95d
83e
96
95 (86)
93
To gain insight into the catalytic nature of 3 and to eliminate
the factors affecting catalytic performance, a few controlled
experiments were investigated. Firstly, we performed a hotꢀ
filtration experiment, eliminating the possible disturbance
coming from residual homogeneous catalyst via a nonꢀ
covalent physical adsorption. In this case, catalyst 3 was filꢀ
tered from the reaction mixture during the ATH of 4ꢀmethoxyꢀ
Nꢀ(2,2,2ꢀtrifluoroꢀ1ꢀphenylethylidene)aniline after 8 h, and the
reaction was then continued to react at 40 °C for another 8 h.
The result showed that 75% yield and 87% ee of chiral prodꢀ
2
Ph
3
Ph
24
55
4
pꢀFPh
pꢀClPh
pꢀBrPh
mꢀBrPh
pꢀCF3Ph
pꢀMePh
pꢀMeOPh
2ꢀthienyl
16
93
5
16
93
98
6
16
91
94
uct
(S)ꢀ4ꢀmethoxyꢀNꢀ(2,2,2ꢀtrifluoroꢀ1ꢀphenylethyl)aniline
7
16
94
96
could be obtained in the first 8 h, but both conversion and ee
value in the second 8 h had no appreciable change (see SI in
Figure S8). This finding suggests that there is no disturbance
coming from residual homogeneous catalyst via a nonꢀ
covalent physical adsorption.
8
12
97
98
9
24
84
98
10
11
16
89
95
Secondly, we utilized a parallel experiment to test the
function of PEG molecule in a homogeneous catalysis system,
indicating the role of PEGꢀfunctionality in catalyst 3 in a
heterogeneous catalysis system. In this case, the ATH of 4ꢀ
28
78
97
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methoxyꢀNꢀ(2,2,2ꢀtrifluoroꢀ1ꢀphenylethylidene)aniline cataꢀ
ly). These observations demonstrated that the designed heteroꢀ
lyzed by its homogeneous MesityleneRuTsDPEN in the presꢀ
ence of PEG molecule were investigated. It was found that the
reaction afforded the corresponding chiral products with 93%
yield and 95% ee, which was obviously better than that atꢀ
tained in the absence of PEG molecule (Entry 2 vs Entry 1 in
brackets). This phenomenon should be due to fact that the
enhanced yield from 86% to 93% was attributed to the phaseꢀ
transferꢀfunction of the PEG molecule in this homogeneous
catalysis system, suggesting that the PEGꢀfunctionality in the
heterogeneous catalyst 3 should function as the similar role in
the case of 3-catalyzed asymmetric reaction in water.
geneous catalyst 3 possessed a highly efficient asymmetric
transfer hydrogenation of αꢀtrifluoromethylimines, where the
combined functionalities, such as wellꢀdefined chiral singleꢀ
site active species verified by 13C CP/MAS NMR spectra, uniꢀ
formly distributed ruthenium center confirmed by SEM image,
highly dispersive nature in water indicated at Figure 2d, as
well as benefit of PEGꢀfunctionality in silicate network disꢀ
cuss above, made their cooperative contributions in its catalytꢀ
ic performance in water.
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
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24
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31
32
33
34
35
36
37
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42
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44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
100
80
60
PMP
CF3
PMP
CF3
40
20
0
N
HN
Catalyst
HCOONa
Ph
Ph
Catalyst 3
MesityleneRuTsDPEN
Its analogue 3'
FIGURE 4. Reusability of catalyst 3 using 4ꢀmethoxyꢀNꢀ(2,2,2ꢀ
trifluoroꢀ1ꢀ(4ꢀfluorophenyl)ethylidene)aniline as a substrate.
Another important consideration for the design of catalyst 3
is its ease of separation by centrifugation, and the recycled
catalyst can retain its catalytic activity and enantioselectivity
after multiple recycles. It was found that catalyst 3 could be
recovered through highꢀspeed centrifugation and recycled
repeatedly. As shown in Figure 4, in seven consecutive reacꢀ
tions, catalyst 3 still afforded 95% conversion and 96% ee in
0
4
8
12
16
20
24
Reaction time (h)
FIGURE 3. Comparison of the asymmetric transfer hydrogenation
of 4ꢀmethoxyꢀNꢀ(2,2,2ꢀtrifluoroꢀ1ꢀphenylethylidene)aniline
catalyzed by 3, its analogue 3', and the homogeneous
MesityleneRuTsDPEN. Reactions were carried out at substrateꢀ
toꢀcatalyst mole ratio of 50 in water.
the
ATH
of
4ꢀmethoxyꢀNꢀ(2,2,2ꢀtrifluoroꢀ1ꢀ(4ꢀ
fluorophenyl)ethylidene)aniline (see SI in Table S1 and in
Figure S9).
Finally, we employed a comparable SiO2ꢀbased Mesityleneꢀ
RuArDPENꢀfunctionalized inorganosilicate analogue as a
parallel catalyst (this inorganosilicate analogue 3' has no PEGꢀ
functionality in its silicate network), judging the function of
PEGꢀfunctionality in catalyst 3. In the case, its comparable
analogue 3' was synthesized through the selfꢀtemplating asꢀ
sembly of POES5a and 2 followed by the similar complexation
with [RuCl2(mesitylene)]2 (see SI in experimental and Figure
S6). The results showed that the ATH of 4ꢀmethoxyꢀNꢀ(2,2,2ꢀ
trifluoroꢀ1ꢀphenylethylidene)aniline catalyzed by its analogue
3' within 24 h only afforded the corresponding chiral products
in 55% yield with 83% ee (Entry 3). This observation suggestꢀ
ed that the absence of PEGꢀfunctionality in the silicate nework
of 3' was responsible for its poor catalytic performance,
confirming the ability of PEGꢀfunctionality in catalyst 3. A
direct evidence could be offered to support this view through a
kinetic investigation in the ATH of 4ꢀmethoxyꢀNꢀ(2,2,2ꢀ
trifluoroꢀ1ꢀphenylethylidene)aniline. As shown in Figure 3,
SCHEME 2. Preparation of the heterogeneous catalyst 5.
More importantly, by utilizing amphiphilic PEGꢀPEOS
silica precursor as a selfꢀtemplating assembled platform, it is
also convenient to immobilize others chiral molecules for conꢀ
struction of functionalized heterogeneous catalysts. As shown
in Scheme 2, chiral cinchonineꢀbased squaramideꢀ
functionalized mesostructured nanoparticles, abbreviated as
SquaramideꢀMSNs (5) (chiral squaramide (4):13 3ꢀ((3,5ꢀ
bis(trifluoromethyl)benzyl)amino)ꢀ4ꢀ(((1S)ꢀ(6ꢀ
the
ATH
of
4ꢀmethoxyꢀNꢀ(2,2,2ꢀtrifluoroꢀ1ꢀ
phenylethylidene)aniline catalyzed by catalyst 3, by its homoꢀ
geneous MesityleneRuTsDPEN and by its inorganosilicate
analogue 3' under the same reaction conditions were perꢀ
formed. It was found that the reaction catalyzed by catalyst 3
had an initial activity (TOF value) remarkedly higher than that
achieved with its analogue 3', even higher than that of its hoꢀ
mogeneous counterpart MesityleneRuTsDPEN (the initial
TOFs within 2 h were 6.0, 2.0 and 4.0 molmolꢀ1hꢀ1, respectiveꢀ
methoxyquinolinꢀ4ꢀyl)((2S)ꢀ5ꢀvinylquinuclidinꢀ2ꢀ
yl)methyl)amino)cyclobutꢀ3ꢀeneꢀ1,2ꢀdione), could also be
fabricated steadily through this selfꢀtemplating assembled
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approach. Similar to the preparation of catalyst 3, the selfꢀ
10
3ꢀOMe
2ꢀOMe
20
20
95
93
98
96
templating assembly of (triethoxysilyl)propanethiol and amꢀ
phiphilic PEGꢀPEOS (1) led to the mercaptoꢀfunctionalized
mesostructured nanoparticles (SHꢀMSNs), which the reaction
with chiral squaramide (4) by a thiolꢀene click reaction14
produced the heterogeneous catalyst 5 as a white powder (see
SI in Figure S1, S3ꢀS7). A benefit of this heterogeneous cataꢀ
lyst 5 was that it enabled a highly efficient asymmetric
Michael addition of acetylacetone to nitroalkenes in brine,
where various substrates could be transferred smoothly to the
responding chiral products within twenty minute reaction time
as shown in Table 2. Taking the asymmetric Michael addition
of acetylacetone to nitrostyrene as an example, it was found
that the enantioselective reaction catalyzed by catalyst 5
afforded (S)ꢀ3ꢀ(2ꢀnitroꢀ1ꢀphenylethyl)pentaneꢀ2,4ꢀdione in
95% yield with 99% ee value, which was comparable to that
of its homogeneous counterpart (entry 1 versus 1 in the
bracket).13c As expected, the highly dispersed chiral active
centers and phaseꢀtransferꢀfeatured PEGꢀfunctionality could
enhance cooperatively enantioselective Michael addition of
acetylacetone to nitroalkenes in aqueous medium. Furthermore,
it was found that the structural and electronic properties did
not affect significantly their enantioselectivities regardless of
electron–withdrawing and electron–donating substituents in
the Ar moiety at R3 group (Entries 2–11). Also, catalyst 5
could be recovered through highꢀspeed centrifugation and
recycled repeatedly, where the recycled catalyst 5 still afforded
92% conversion and 97% ee after a continuous eight times
recycle in the enantioselective Michael addition of
acetylacetone to nitrostyrene (see SI in Tabel S2 and in Figure
S11).
3
4
5
6
7
8
9
11
a
Reaction conditions: catalyst 5 (36.26 mg, 5.0 ꢁmol of squaramide
based on TG analysis), nitroalkenes (1.0 mmol), acetylacetone (2.0 mmol),
5.0 mL brine, reaction time (20 minute). Determined by chiral HPLC
analysis (see SI in Figure S10).
homogeneous squaramide as a catalyst.
b
c
Data were obtained using its
10
11
12
13
14
15
16
17
18
19
20
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22
23
24
25
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28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
In conclusions, we utilize a selfꢀtemplating assembled apꢀ
proach to develop two functionalized mesostructured silicas as
heterogeneous catalysts. As presented in this study, chiral
ruthenium/diamineꢀfunctionalized heterogeneous catalyst disꢀ
plays high catalytic activity and enantioselectivity in the
asymmetric transfer hydrogenation of αꢀtrifluoromethylimines
to chiral aryltrifluoromethylamines, while chiral squaramideꢀ
functionalized heterogeneous catalyst enables asymmetric
Michael addition of acetylacetone to nitroalkenes in brine to
afford chiral products with highly catalytic efficiency. As deꢀ
signed, the combined functionalities, such as wellꢀdefined
singleꢀsite active species, highly dispersed nanoparticles, as
well as phaseꢀtransferꢀfunction PEGꢀfunctionality, contribute
cooperatively the catalytic performance in an environmentally
friendly medium. Furthermore, both catalysts could be recovꢀ
ered and reused repeatedly without any obvious effect on its
catalytic performance. The outcomes from the study suggest
that a simple selfꢀtemplating assembled approach could be
used to construct various chiral organomoleculeꢀ
functionalized mesostructured silicas for highly efficient
asymmetric catalysis.
TABLE 2. Asymmetric Michael addition of acetylacetone to
nitroalkenes.a
ASSOCIATED CONTENT
Supporting Information
Experimental procedures and analytical data of chiral products are
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Time
Yield
Ee.
Corresponding Author
Entry
R1
(minute) (%)b
(%)b
ghliu@shnu.edu.cn
1
2
3
4
5
6
7
8
9
4ꢀH
20
20
20
20
20
20
20
20
20
95 (98)
99(99)c
99
ACKNOWLEDGMENT
We are grateful to China National Natural Science Foundation
(21402120), and Shanghai Sciences and Technologies Developꢀ
ment Fund (13ZR1458700), Shanghai Municipal Education
Commission (14YZ074) for financial supports.
4ꢀF
97
96
95
93
94
97
95
97
4ꢀCl
97
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3ꢀCl
99
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Graphical Abstract：
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Amphiphilic Hyperbranched Polyethoxysiloxane: A
Self-Templating Assembled Platform to Fabricate
Functionalized Mesostructured Silicas for Aqueous
Enantioselective Reactions
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Kun Zhang, Juzeng An, Yanchao Su, Jueyu Zhang,
Ziyun Wang, Tanyu Cheng, Guohua Liu *
Two amphiphilic mesostructured silicaꢀbased heterogeꢀ
neous catalysts are fabricated through a selfꢀtemplating
assembled strategy, which enable highly efficient enanꢀ
tioselective reactions in an environmentally friendly meꢀ
dium.
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